In the use of heat pumps, it is extremely desirable that the displacement of the compressor of such a heat pump be reduced to optimize the heat pump annual performance factor in the Northern Hemisphere. This desired advantage follows from the conflict between needed reduction in capacity balance temperature (heating) to avoid excessive use of resistance heat on the one hand avoiding excessive loading of heat exchangers with excess capacity in mild day heating and summer cooling operations.
Conventional two-speed motors are limited to 50% capacity reduction which, due to evaporator-compressor capacity feedback effects, provides insufficient capacity control with a considerable cost penalty in using such a two-speed motor. A need has, therefore, arisen for improved drive apparatus for achieving the aforesaid advantage without the expense involved in the use of conventional two-speed motors.